Process for recovery of oil



Sept 13, 1966 MORSE PROCESS FOR RECOVERY OF OIL Filed Dec. 15, 1963 15INVENTOR.

lQ/Cf/AQD .4. MOQSE ATTORNEY.

United States Patent 3,272,261 PROCESS FOR RECOVERY OF OIL Richard A.Morse, Oakmont, Pa., 'assignor to Gulf Research & Development Company,Pittsburgh, Pa., a corporation of Delaware Filed Dec. 13, 1963, Ser. No.330,497 11 Claims. (Cl. 16611) This invention relates to the recovery ofviscous oils from subsurface oil-bearing formations, and moreparticularly to a method of stimulating a well for production of suchoils.

There are a number of known underground reservoirs containing very largeamounts of oil from which there has been little if any productionbecause of the high viscosity of the oil. The high viscosity of the oilfrequently does not prevent flow through the underground formation atvery low velocities; however, the radial flow pattern into wells makesnecessary relatively high velocities adjacent production wells andprevents production of highly viscous oil at profitable rates eventhough the permeability of the formation may be high.

Attempts have been made to stimulate production from wells penetratingoil-bearing formations containing highly viscous oils by injecting heatfrom the wells through which oil is to be produced into the surroundingformation to reduce the viscosity of the oil near the well. Frequently,the well is damaged because of excessive temperatures, or other reasons,which cause increased resistance to flow into the well. Moreover,processes for heating the formation surrounding a well from the wellthrough which the oil is to be produced are necessarily of anintermittent nature in which a heating period is followed by a producingperiod, which in turn must be followed by another heating period iffurther production is to 'be obtained. It is apparent that the well willtherefore only be on production part of the time.

ln-situ combustion of the oil in the formation by the injection of anoxygen-containing gas into the formation at an injection well spaced asubstantial distance from the production well has been suggested as amethod of recovering highly viscous oils. The injection well in suchprocesses is located approximately at the boundary of the drainage areaof the production well. One of the purposes of in-situ combustionprocesses is to supply heat to the formation by combustion of the oil toraise the temperature of the formation and thereby reduce the viscosityof the oil in the formation.

Two types of in-situ combustion processes have been developed. In thereverse burning process, an oxygencontaining gas is injected into theformation at the injection well and oil in the formation is ignited atthe production well. The combustion front travels countercurrently tothe flow of oxygen-containing gas, and oil is driven from the region ofthe combustion front through the heated formation to the combustionwell. Although reverse burning processes quickly heat the formationaround the production well and thereby reduce the viscosity of the oilflowing into the production well, the process has several disadvantages.A substantial portion of the oil in the reservoir is converted to cokewhich cannot be recovered. Reverse burning is thermally inefficient; atleast partly because the entire formation between the injection well andproduction well is hot at the end of the process. It is necessary toburn enough oil in the formation to raise the temperature of the entireformation as well as the oil produced.

In the forward burning process, oil in the formation is ignited adjacentthe well from which the oxygen-containing gas is injected into theformation and the combustion front moves through the formation towardthe production well as the forward burning continues. The temperaturegradient ahead of the combustion front is steep; hence, a bank of coldoil must be displaced through the formation ahead of the combustionfront. It is apparent that until the combustion front is near theproduction well nothing has been done to reduce the viscosity of the oilflowing into the production well and the resistance to such flow isextremely high. Frequently, the resistance to flow of the bank of coldoil is high enough to prevent injection of an oxygen-containing gas atrates adequate to maintain combustion of the oil in the reservoir.

The amount of oil present in the reservoir in a forward combustionprocess between the first heated oil to appear at the production welland the combustion front is small, and no oil is present in theformation behind the combustion front. Once there is a substantialincrease in temperature in the production well, for example, to atemperature of 200 or 300 F., injection of air is stopped in the usualforward burning process. Further injection of air merely results inchanneling of hot air to the production well. The resultant very hightemperatures in the production well causes serious corrosion in thatwell.

This invention resides in a method of recovering viscous oils fromsubsurface oil-bearing formations in which a heater well is drilled intothe formation a distance in the range of about 50 to feet from aproduction well, and well within the drainage area of the productionwell. Heat is injected into the formation at the heater well until thebottom hole temperature of the production well increases. The rate ofheat injection at the heater well is then reduced to a rate adapted tomaintain the bottom hole temperature at the production well in the rangeof about 200 to 600 F. and the injection of heat at the heater well iscontinued at the lower rate while continuing production of oil throughthe production well.

In the drawings:

FIGURE 1 is a diagrammatic plan view of a production well with a heaterwell adjacent thereto showing temperature contours at breakthrough ofheat into the production well and during subsequent production inaccordance with this invention;

FIGURE 2 is a diagrammatic view taken along section line 22 in FIGURE 3of an embodiment of this invention in which heat is injected into theformation at a heater well simultaneously with the injection of a drivefluid into the oil-bearing formation at an injection well located at adistance from the production well substantially greater than thedistance between the production well and the heater well; and

FIGURE 3 is a diagrammatic plan view of the embodiment of the inventionillustrated in FIGURE 2.

The process of this invention is of particular value in increasing therate of production of oil from reservoirs containing oil having aviscosity higher than 50 centipoises at normal reservoir temperatures.In some reservoirs, for example at Yorba Linda, California, the oil hasa viscosity of 8000 to 9000 centipoises at reservoir temperature.Greatest benefits from the process are obtained when the viscosity ofthe oil in the reservoir decreases rapidly with an increase intemperature of the oil. Because this invention relies on the flow of oilfrom the surrounding cold area across the boundary of the heated area,it is no more effective than a normal forward burning process inreservoirs containing oil having a pour point higher than reservoirtemperature. Because the advantages of this process increase with theextent to which oil flowing into the formation is heated, and themaximum heating is limited by cracking of the oil to form coke whichbecomes serious at temperatures above 600 F., this process is mostuseful in recovering oil from reservoirs at a temperature below 200 F.,and especially from reservoirs at a temperature below 150 F., containingoil having a viscosity less than centipoises at 600 F.

A production Well is drilled into the oil-bearing formation and a heaterWell is drilled into the oil-bearing formation at a distance rangingfrom 50 to 100 feet from the production well. well is drilled from theproduction well will be determined by the characteristics of theformation; principally the viscosity of the oil and the permeability ofthe formation. The distance from the production well to the heater wellshould be such that an increase in bottom hole temperature of theproduction well should be obtained within no more than two weeks afterinjection of heat at the heater well commences. Both the production welland the injection well can be completed in a conventional manner.Ordinarily, casing is run and cemented through the oil-bearingformationin each of the wells, and followed by perforation of thecasing. It is preferred, especially when the injection of heat is by theinjection of air for in-situ combustion, to complete the heater well ina manner to restrict the injection of heat to a narrow zone near thebase of the oil zone to avoid channeling across the top of the oil zone.

The heat injected into the oil-bearing formation at the heater well canbe derived from any of several sources. When the oil-bearing formationis relatively shallow and at a relatively low pressure, the heat iseconomically supplied at the heater Well by the injection of steam fromthat well into the oil-bearing formation. Another method of injectingheat into the formation is by burning a fuel in a down-hole burner,which may be of the type described in US. Patent No. 2,668,592 of Piroset al., and displacing combustion products from the heater well into thesurrounding oil-bearing formation. Another method of injecting heat intothe formation is to displace an oxygencontaining gas down the heaterwell and into the formation, ignite oil in the formation adjacent theheater well, and continue the injection of the oxygen-containing gaswhereby the continued combustion of oil liberates heat in the formation.When an oxygen-containing gas is injected into the formation to supplyheat by in-situ combustion, the gaseous products of combustion flow tothe production well and aid lifting oil through that Well to thesurface. An advantageous method of supplying heat to the reservoir is toinject a mixture of steam and air; the steam supplies the major portionof the heat and the gaseous combustion products aid in lifting oil inthe production well.

Heat is injected into the formation at the heater well at a high ratesuch that a bottom hole temperature rise will occur at the productionwell in less than two weeks, and preferably within two or three daysafter injection of heat is commenced. In reservoirs in which thepermeability of the formation is low and the oil has a high viscosity,it may be necessary to fracture the formation from the heater well tothe production well before heat can be injected into the formation atthe desired high rate. If the injection of heat at the heater well isaccomplished by the injection of air to cause in-situ combustion of oilin the oil-bearing formation, the air can be injected at a rate of, forexample, 1,000,000 to 5,000,000 std. cu. ft. per day during the initialheating step. During that step, the production well may be shut inwhereupon the flow of heat from the heater well will be substantiallyuniform in all directions from the heater well except for variationscaused by variations in the permeability of the formation. The initialstep of injecting heat at a high rate into the formation at a heaterwell also can be performed simultaneously with production of oil fromthe production well. In that event, the flow of heat from the heaterwell will proceed substantially uniformly in all directions with Thedistance at which the heater variations determined by permeabilityvariations until the outer boundary of the heated zone is close enoughto the production well for the reduced pressure surrounding theproduction well to influence the flow pattern in the subsurfaceformation.

Referring to FIGURE 1 of the drawings, a production well 10 isillustrated having a heater well 12 spaced to feet from the productionWell 10. If the embodiment of the invention illustrated in FIGURE 1, oilis produced at the production well 10 during the initial injection ofheat at the heater well 12; hence, the boundary 14 of the heated zone isflattened near the production well 10.

Upon an increase in the'bottom hole temperature at the production well10, the rate of injection of heat into the formation at the heater wellis reduced. The reduction in the rate of heat injection can be made bystopping all injection of heat at the heater well until a temperaturepeak has been reached in the production well, and thereafter injectingheat at the heater well at a rate to maintain a bottom hole temperaturein the production well in the range of 200 to 600 F. Another method ofobtaining the desired control of bottom hole temperature in theproduction well is merely to decrease the rate of heat injection andthen make Whatever further adjustments in the rate of heat injection atthe injection well are necessary to obtain the desired bottom holetemperature in the production well. The reduced rate of injection ofheat is less than about one-half the initial rate, and in most instancesless than one-fourth the initial rate. For example, in a typicalapplication of this invention using in-situ combustion to supply theheat, air is injected into the formation at a heater well spaced fiftyfeet from a production well at a rate of approximately 2,500,000 std.cu. ft. per day during the initial heating step, and after a temperaturerise occurs at the production well, the rate of air injection at theheater well is reduced to approximately 500,000 std. cu. ft. per day.

The rate of injection of heat at the heater well after breakthrough ofheat at the production well is controlled to maintain the bottom holetemperature at the production well at a level such that oil in theformation flows readily to the production well. If the viscosity of theoil in the reservoir is only slightly over 50* centipoises at reservoirtemperature and the permeability of the formation is relatively high,only slight heating will be necessary to stimulate production from theproduction well. If a satisfactory rate of production can be obtained ata production well temperature below F., the benefits of this processusually are not sufficient to justify using the process. In reservoirswhere this process is most attractive, it will usually be desirable toinject heat at the heater well at a rate such that the temperature ofthe oil is just below the temperature at which substantial crackingoccurs. Usually cracking is not excessive unless the temperature at theproduction well exceeds 600 F. If air is injected into the formation ata heater well to supply heat by in-situ combustion, it is desirable toreduce the rate of injection of air at the heater well to a rate whichresults in the appearance of substantially no free oxygen at theproduction well. A limitation on the minimum rate of injection of heatwhen the heat is in the form of air for in-situ combustion is that theair must be injected at a rate adequate to maintain high temperaturecombustion of oil in the reservoir.

It is advantageous in the process of this invention to reduce the bottomhole pressure in the production well and the pressure at which a heatingmedium is injected at the heater well to as low a level as possibleafter the temperature rise occurs at the production well. The areaaround the production well then becomes a pressure sink into which oilfrom the surrounding formation flows.

After the breakthrough of heat into the production well, there is atendency for the heating medium injected at the heater well to channeltoward the production well. The boundary of the heated zone willthenvcontract to a position such as is shown in FIGURE 1 by the line 16.During continued injection of heat at the heater well and continuedproduction of oil from the production well 10, oil from the surroundingformation flows across the boundary 16 into the heated zone of theformation and is further heated by the heat injected at the heater well12 as the oil flows to the production well 10. As indicated in FIGURE 1,the boundary 16 of the heated zone has an area 100 times or more thearea of the borehole of production well 10. Because of the very largearea of the boundary 16, the total flow of cold oil from the surroundingformation into the heated zone is large. The reduced viscosity of theoil within the boundary 16 allows that oil to flow readily to productionwell and is lifted through that well to the surface.

It is an important advantage of this invention that the breakthrough ofheat to the production well and, hence, the increased rate of productionoccur very quickly after heat injection is begun. Unlike the normalforward burning process in which an oxygen-containing gas is injected atan injection well near the boundary of the production wells drainagearea, production of oil continues after the breakthrough of heat to theproduction well. Control of the rate of heat injection allows control oftemperature in the production well and the intervening formation toprevent excessive temperatures which cause the deposition of coke withinthe formation.

In the embodiment of the invention illustrated in FIG- URES 2 and 3 ofthe drawings, the use of a heater well is combined with the injection ofa drive fluid at a remote injection well spaced from the production wellthe normal distance employed in ordinary secondary recovery operations,i.e., near the boundary of the drainage area of the production well. Thedrive fluid aids in moving oil from that portion of the oil-bearingformation between the heater well and the injection well into the heatedzone between the heater well and the production well. Water drive, gasdrive, or in-situ combustion can be used to supply energy to move oilthrough the formation from the vicinity of the injection well to theheated zone.

Referring to FIGURE 2, a production well, indicated generally by 18, isdrilled through overlying formations into an oil-bearing formation 20containing a viscous oil. Well 18 is illustrated having a packer 22 settherein and a tubing string 24 run through the packer for the deliveryof oil upwardly through the well.

A heater well indicated generally by reference numeral 26 is drilledinto formation 20 at a distance approximately 50 to 100 feet from thewell 18. An injection well 28 is drilled into formation 20 at a distancefrom the production well 18 normal for an injection well in a secondaryor pressure maintenance operation. Injection well 28 will ordinarily beat least six times as far from the production well 18 as the heaterwell; a distance of 300 to 1000 feet between the injection well andproduction well is normal. The heater well 26 is between the productionwell 18 and injection well 28. Although the heater well preferably issubstantially on a line connecting those two wells, as shown in FIGURE 3of the drawings, it is only necessary that it be generally between thewells. The line from the heater well to the production Well may make anangle of as much as 90 with the line joining the production andinjection wells, but, preferably, makes an angle of 60 or less with suchline. Wells 26 and 28 are illustrated with packers 30 and 32,respectively, set therein and tubing strings 34 and 36 run through thepackers. Casing of well 26 is perforated in a narrow zone near thebottom of the oil-bearing formation 20 to minimize channeling across thetop of that formation to the production well.

In the operation of the embodiment of the invention illustrated inFIGURES 2 and 3, a drive fluid, which for the purposes of description ofthe invention is water, is injected through tubing 36 downwardly in well28 and discharged into the oil-bearing formation 20. Meanwhile air isinjected at a high rate down well 26, and oil in the formation 20surrounding well 26 is ignited by, for example, the displacement of apyrophoric material outwardly into the formation ahead of the air.Injection of the air is continued at the heater well 26 for a periodadequate to form a heated zone extending from the heater well 26 beyondthe production well 18. The boundary of the heated zone is indicated byline 38 in FIGURE 3 of the drawings.

After the bottom hole temperature in production well 18 increases,production from production well 18 is commenced and the rate ofinjection of air at the heater well 26 is reduced to less than one-halfthe initial rate to control the bottom hole temperature in theproduction well between 200 and 600 F. The boundary of the heated zonemoves inwardly to a position such as indicated by line 40 in FIGURE 3.Injection of air at the heater well 26 is continued at the reduced ratesimultaneously with the production of oil through well 18. Oil is driventhrough the formation by the drive fluid injected at the injection well28 ahead of interface 42 between the drive fluid and oil and enters theheated zone around the heater well. Because of its reduced viscosityresulting from the heating, the oil flows readily to the production well18.

The injection of heat into a formation containing a viscous oil at aheater well spaced a short distance from a well through which oil isproduced from the formation has the effect of greatly extending theeffective diameter of the production well. Because of the greatlyreduced viscosity of the oil after it enters the heated zone, theresistance to flow through the heated zone of the formation is low. Thevery large area of the boundary between the heated zone and thesurrounding cold formation allows substantial total flow rates into theheated zone even though the oil flowing into the heated zone has anextremely high viscosity. By injecting heat into the formation at aheater well spaced 50 to feet from a production well, the critical areaof the formation around the production well can be continuously heatedto stimulate production from the reservoir while oil drains from a largearea into the heated zone. By maintaining a low pressure in theproduction well after the temperature rises in that well, and injectingfluids supplying heat at a low rate at the heater well, the entireheated zone becomes a low pressure sink of large area; therebyfacilitating flow of cold oil into the heated zone and mixture of thecold oil with hot oil in that zone.

I claim:

1. A method for recovery of oil from an underground oil-bearingformation penetrated by a production well, an injection well spaced fromthe production well, and a heater Well between the production well andinjection well and near the production well comprising drilling saidheater well within the radius of drainage of the production well at adistance of from about 50 to about 100 feet from the production well,injecting a drive fluid at the injection well to drive oil through theformation toward the production well, injecting heat into the formationat the heater well at a rate sufficient to raise the bottom holetemperature in the production well to a temperature of at least 200 F.within a period of two weeks after commencing injection of heat at theheater well, thereby creating in the formation a zone having atemperature greater than 200 F. around the production well, afterbreak-through of heat at the production well reducing the rate of heatinjected at the heater well to maintain a bottom hole temperature in therange of 200 to 600 F. at the production well, and continuing theinjection of drive fluid at the injection well and the injection of heatat a reduced rate at the heater well while producing oil from theformation through the production well.

2. A method as set forth in claim 1 in which the oil in the formationhas .a viscosity exceeding 50 centipoises at the formation temperature.

3. In a process for increasing the production of oil from an oil-bearingformation penetrated by a production well and a heater well spaced fromthe production well, the improvement comprising drilling the heater wellwithin the drainage area of the production well at a distance of fromabout 50 to about 100 feet, injecting heat into the formation at theheater well at a rate sufficient to raise the bottom hole temperature inthe production well to at least 200 F. within two Weeks after commencinginjection of heat at the heater well to form .a zone having atemperature greater than 200 F. around the production Well, thereafterreducing the rate of injecting heat into the formation at the heaterwell, producing oil from the production well, and continuing theinjection of heat into the formation at the heater well while producingoil at the production well to maintain the temperature in the productionwell in the range of 200 to 600 F.

4. A process as set forth in claim 3 in which the oil in the oil-bearingformation has a viscosity exceeding 50 centipoises -at the formationtemperature.

5. In a process for increasing the production of oil from an oil-bearingformation penetrated by a production well, an injection well, and aheater well between said production well and said injection well, theimprovement comprising drilling said heater well within the drainagearea of the production Well at a distance within 50 to 100 feet from theproduction well, said injection well being at least six times as faraway from the production well as is the heater well, injecting heat intosaid oil-bearing formation at the heater well at an initial ratesufficient to raise the bottom hole temperature in the production wellto at least 200 F. within two weeks after commencing the injection ofheat at the heater well to form a zone in the formation having atemperature greater than 200 F. around the production well, after heatfrom the heater well reaches the production well reducing the rate ofinjecting heat into the formation at the heater Well to a rate less thanone-half the initial rate to maintain the temperature in the productionwell in the range of 200 to 600 F. and maintain said heated zone aroundthe production well, and injecting a fluid into the oil-bearingformation at the injection well to drive oil through the formation tosaid heated zone,

6. A process -as set forth in claim 5 in which the injection of heat atthe heater well is accomplished by the injection of air to effectin-situ combustion in the formation.

7. A process as set forth in claim 5 in which the pressure in theproduction well is reduced after heat from the heater well reaches saidproduction well.

8. In a process for increasing production of oil from an oil-bearingformation penetrated by a production well and a heater well, theimprovement comprising drilling said heater well within the drainagearea of the production well at a distance of from about 50 to about 100feet from the production well, injecting air into the formation at theheater well, igniting oil in the formation adjacent the heater well andcontinuing the injection of air into the formation at an initial rate atthe heater well sufiicient to raise the bottom hole temperature in theproduction well to at least 200 F. within two weeks after commencing theinjection of air in the heater well to form .a heated zone in theformation having a temperature greater than 200 F. around the productionwell, simultaneously with the injection of air at the heater wellproducing oil at the production well, after an increase in temperaturein the production well discontinuing the injection of air at the heaterwell until the temperature in the production wel-l passes a peak, theninjecting air into the formation at the heater well at a rate less thanone-half the initial air injection rate controlled to maintain thetemperature in the production well in the range of 200 to 600 F.

9. In a process for increasing the production of oil from an oil-bearingformation penetrated by a production well and a heater well, theimprovement comprising drilling said heater well within the drainagearea of the production well at a distance of from about 50 to about feetfrom the production well, creating a fracture extending from the heaterwell to the production well, injecting a heating medium into theoil-bearing formation at the heater well at a rate sufiicient to raisethe bottom hole temperature in the production well to at least 200 F.within two Weeks after commencing injection of the heating medium at theheater well to form in the formation a heated zone having a temperaturegreater than 200 F. around the production well whereby thetemperature inthe production well increases, reducing the rate of injecting theheating medium at the heater well to a rate not exceeding one-half theinitial rate upon an increase in temperature of the production well, andcontinuing the injection of the heating medium while producing oilthrough the production well.

10. A method for recovery of oil from an underground oil-bearingformation penetrated by a production well, an injection well remotelyspaced from the production well and a heater well adjacent theproduction well, said method comprising drilling the heater well withinthe drainage area of the production well at a distance of from about 50to about 100 feet from the production well, injecting a drive fluid atthe injection well to drive oil through the formation toward theproduction well, injecting air into the formation at the heater well ata rate of from about one million to about five million standard cubicfeet per day, igniting oil in the formation adjacent said heater welland continuing the injection of air thereby creating in the formationaround the production well a zone having a temperature greater than thenormal formation temperatur e after break-through of heat at theproduction well reducing the rate of air injection at the heater well tomaintain a bottom hole temperature in the production well within therange of 200 to 600 F., and continually injecting drive fluid at theinjection well and continually injecting air at a reduced rate at theheater well while producing oil from the formation through theproduction well. i I

11. A method for recovery of oil from an underground oil-bearingformation penetrated by a production well, an

injection well remotely spaced from the production well and a heaterwell near the production well, said heater well being located a distanceof about 50 to 100 feet from the production well, said method comprisinginjecting a drive fluid at the injection Well to drive oil through theformation toward the production well, injecting air into the formationat the heater well, igniting oil in the formation adjacent the heaterwell, continuing the injection of air int-o the formation at theheaterwell at a rate adequate to maintain the temperature of the formation oilat a temperature sufficient to support in situ combustion of that oiland create in the formation around the production well a zone having atemperature greater than the normal formation temperature,simultaneously with the injection of the drive fluid at the injectionwell and the injection of air at the heater well producing oil at theproduction well, after an increase in temperature at the production wellreducing the rate of air injection at the heater well and controllingsaid rate of air injection to maintain the bottom hole temperature inthe production well within the range of from about 200 to about 600 F.

References Cited by the Examiner UNITED STATES PATENTS 2,695,163 11/1954Pearce et a1 l66-ll 2,841,375 7/1958 Salomonsson 166-11 X 2,862,55812/1958 Dixon 166-11 X 2,946,3 82 7/1960 Tek et al 166 11 (@therreferences on following page) 1 9 UNITED STATES PATENTS Crawford et a1166-11 Frey 166--11 Willrn'an 1661 1 5 West et a-l 166-11 Kerr 166--1110 OTHER REFERENCES McNiel, Jr., et a1.: Oil Recovery By In-SituCombustion, The Petroleum Engineer, July 1958, pp. B-29-32, 36, 41 and42.

CHARLES E. OCONNELL, Primary Examiner. JACOB L. NACKENOFF, Examiner. S.J. NOVOSAD, Assistant Examiner.

3. IN A PROCESS FOR INCREASING THE PRODUCTION OF OIL FROM AN OIL-BEARING FORMATION PENETRATED BY A PRODUCTION WELL AND A HEATER WELL SPACED FROM THE PRODUCTION WELL, THE IMPROVEMENT COMPRISING DRILLING THE HEATER WELL WITHIN THE DRAINAGE AREA OF THE PRODUCTION WELL AT A DISTANCE OF FROM ABOUT 50 TO ABOUT 100 FEET, INJECTING HEAT INTO THE FORMATION AT THE HEATER WELL AT A RATE SUFFICIENT TO RAISE THE BOTTOM HOLE TEMPERATURE IN THE PRODUCTION WELL TO AT LEAST 200*F. WITHIN TWO WEEKS AFTER CONNECTING INJECTION OF HEAT AT THE HEATER WELL TO FORM A ZONE HAVING A TEMPERATURE GREATER THAN 200*F. AROUND THE PRODUCTION WELL, THEREAFTER REDUCING THE RATE OF INJECTING HEAT INTO THE FORMATION AT THE HEATER WELL WHILE PRODUCING OIL AT DUCTION WELL, AND CONTINUING THE INJECTION OF HEAT INTO THE FORMATION AT THE HEATER WELL WHILE PRODUCING OIL AT THE PRODUCTION WELL TO MAINTAIN THE TEMPERATURE IN THE PRODUCTION WELL IN THE RANGE OF 200* TO 600*F. 